1. Field of the Invention
The present invention relates to differential amplifiers, and in particular, to differential amplifiers with a controllable common mode voltage.
2. Description of the Related Art
Signal amplifiers are used in many applications and may include amplifiers used for buffering or otherwise amplifying AC signals, as well as buffering or otherwise amplifying DC signals. One particular example of amplifiers used for DC signals includes those amplifiers used for generating reference voltages. Such reference voltage amplifiers are sometimes required to drive large circuit capacitances, such as those found in many analog signal systems. With such highly capacitive loads, it is difficult to achieve high DC gain and high bandwidth in an efficient manner. One common technique to address this problem is to use a differential amplifier. However, differential amplifiers require that both the input and output common mode voltages be controlled.
One widely used common mode control involves the placement of a voltage divider at the output of the amplifier. However, this reduces the DC gain of the amplifier, thereby requiring the use of a multistage amplifier to compensate for such gain reduction.
Another technique is the use of a multistage, or cascaded, transconductance (gm/gm) amplifier. However, a cascaded transconductance amplifier will still have low DC gain and poor phase margin since the amplifier must be internally compensated. In an effort to maintain DC gain by avoiding the use of a resistive output voltage divider, one technique involves a simple gate-averaging amplifier at the output to sense the output voltage and then apply a feedback voltage to correct the output common mode voltage. However, only the output amplifier stage benefits from this, leaving the auxiliary, or preceding, amplifier stage(s) with no common mode voltage control. As a result, the common mode voltage loop will have low DC gain and low bandwidth.